Field of the Invention
The present invention relates to a printed circuit board in which a land of a semiconductor package and a land of a printed wiring board are bonded to each other by solder, and to electronic equipment including the printed circuit board.
Description of the Related Art
Due to recent improvement of performance of a semiconductor element mounted in electronic equipment, high pin count is required to be achieved with lands (electrodes) of a semiconductor package on which the semiconductor element is mounted to realize a stable and high-speed operation of the semiconductor element. Furthermore, to satisfy demands for smaller and thinner mobile devices, digital cameras and the like, the lands are required to be arranged at a finer pitch to achieve the high pin count with a smaller semiconductor package.
Ball Grid Array (BGA) and Land Grid Array (LGA), with which the lands can be densely arranged in a grid form on a lower surface of the package substrate, are employed to realize a semiconductor package achieving the high pin count with the lands arranged at a fine pitch. The BGA semiconductor package is formed in such a manner that solder balls are mounted on the lands by reflow heating before the semiconductor package is attached on a printed wiring board (mother board), so that electrodes have a certain height.
The BGA semiconductor package is then attached to the printed wiring board on which solder is applied on the lands. Then, the reflow heating is again performed, so that the BGA semiconductor package is mounted on the printed wiring board.
The LGA semiconductor package is directly attached, in a state of the semiconductor package with no solder ball mounted thereon, to a printed wiring board with solder applied on the lands, and then is mounted on the printed wiring board by the reflow heating. As described above, the LGA semiconductor package has no solder ball mounted thereon, and thus advantageously achieves a less height of the printed circuit board. Furthermore, the LGA semiconductor package does not require reflow heating for mounting solder balls at the time of manufacturing a semiconductor package, and thus is advantageously used for a semiconductor device package with low thermal durability to reduce a number of heating processing at the time of manufacturing.
Due to the features described above, the LGA is frequently employed for an image sensor package on which image sensor elements are mounted, the image sensor element being the imaging semiconductor element in a mobile device and a digital camera required to be thin and having low thermal durability. Generally, the image sensor package is a hollow LGA semiconductor package formed by mounting an image sensor element on a package substrate molded in a cavity shape and sealing an imaging surface side with glass.
To reduce attaching of foreign objects to a light receiving unit of the image sensor element, in many cases, a base material of a substrate of the LGA package, used as the image sensor package, is a ceramic material featuring less dust emission compared with an organic substrate. The ceramic material further features high thermal conductivity and thus can advantageously prevent image quality degradation due to thermal noise by discharging heat produced when the image sensor element is operated for a long period of time for, for example, live view shooting and movie shooting.
However, the package substrate using the ceramic material is formed by baking, and thus warpage and undulation are likely to occur on the package substrate due to contraction in the baking. Thus, the flatness of the lands cannot be achieved in the LGA semiconductor package using the ceramic material for the package substrate.
Mounting of the LGA semiconductor package, with low flatness as described above, on the printed wiring board results in a state where some portions have a large distance and some portions have a small distance between the land of the package substrate and the land of the printed wiring board facing each other. When the reflow heating is performed in this state, the resultant molten solder is extended at the portion where the distance between the lands is large. The extended molten solder produces force of reducing a surface area due to the surface tension. That is, force of reducing the distance between the package substrate and the printed wiring board occurs. The solder at the portion where the distance between the lands is large is likely to be torn in the heating processing.
Furthermore, the force of reducing the distance between the package substrate and the printed wiring board is multiplied by the number of extended molten solder, so that the force becomes large as a whole. As a result, the molten solder, at the bonding portion with a small distance between the lands, is crushed by the sum of the force of reducing the distance between the package substrate and the printed wiring board produced by the extended molten solder and the load of the semiconductor package. As a result, the molten solder spreads out from the lands. Furthermore, the solder at such portion is easily flattened due to the small distance between the lands. The molten solder spreading outside of lands and coming into contact with the solder at the adjacent lands results in a solder bridging failure after the solder is solidified.
All things considered, the solder bridging failure is likely to occur when a warped or undulated semiconductor package is bonded to a printed wiring board. Japanese Patent Application Laid-Open No. 2005-11921 discloses a method of addressing this problem. More specifically, in the method, a larger printed amount of solder paste is provided to a bonding portion where a distance between a package land and a substrate land is large. With the amount of molten solder increased, force applied by the extended solder to reduce the distance between the package substrate and the printed wiring board is reduced. Thus, an attempt to prevent the solder bridging failure due to the flattened solder is facilitated.
Furthermore, in the method described in Japanese Patent Application Laid-Open No. 2005-11921, the printed amount of solder paste at the bonding portion with a large distance between the package substrate and the printed wiring board is large, and thus the solder is less likely to be torn.
To achieve higher sensitivity, a larger number of pixels, and improved moving image capturing function, high pin count and smaller pitch have also been increasingly required in the image sensor package, as in other semiconductor packages, such as an application integrated circuit (ASIC) or a memory. The lands on the ceramic substrate formed by baking have positions largely varied after the manufacturing. Thus, the ceramic substrate needs to be manufactured to have the land with a large area to prevent misalignment at the time of mounting to the printed wiring board. As a result, the distance between the adjacent lands on the package substrate is small. As described above, the LGA semiconductor package with no solder ball mounted thereon is generally used for the image sensor package.
To achieve the molten solder volume with which the force of reducing the distance between the package and the substrate applied by the extended solder as in the method described in Japanese Patent Application Laid-Open No. 2005-11921 at large lands with no solder ball, the solder paste with an extremely large volume needs to be supplied. When a large amount of solder paste is supplied to a portion where the distance between the adjacent lands is small, the solder pastes, at the adjacent lands, are likely to come into contact with each other due to slump of the solder paste by the mounting of parts or reflow heating, and thus the solder bridging failure is likely to occur.
At a portion where the distance between the lands is small, the solder with a large volume has a large diameter when it is molten and thus is likely to be in contact with the solder at the adjacent land thereby causing the risk of the solder bridging failure.
At the bonding portion where the diameter of the lands is large and the distance between the lands is short, to reduce the risk of the solder bridging failure due to causes other than the flattened solder, a solder supplied amount is preferably regulated to such a value that the height of the solder after the bonding becomes 30% or less of the land diameter. Thus, in the semiconductor package used for the image sensor package, it is difficult to supply a large amount of solder.
When the LGA semiconductor package is mounted on the printed wiring board, no solder ball is used, and thus the solder needs to be supplied only with the solder paste. Thus, the amount of flux supplied to the bonding portion is relatively larger in the LGA semiconductor package than in the BGA semiconductor package having solder balls. The solder paste is separated into solder and flux by being molten. If the flux stays in a recess of the solder resist, forming a land of the printed wiring board, might hinder the molten solder from spreading to the lands of the printed wiring board. As a result, an open solder joint failure is likely to occur in the LGA semiconductor package involving a large amount of flux, due to the flux staying in the recess of the solder resist.